An explosion occurred in the distillation kettle at the workshop of the propyzamide project, a subsidiary of Lier Chemical.

In the early hours of November 22, 2018, during a trial run with material loaded, the distillation kettle in the trifluralin production workshop of Guang’an Lier Chemical Co., Ltd. (hereinafter referred to as “Guang’an Lier”), a wholly owned subsidiary of Lier Chemical Co., Ltd. (hereinafter referred to as “Lier Chemical”), exploded, resulting in four minor injuries and direct economic losses of approximately RMB 1 million.
　　Following the accident, Guang’an Lier promptly activated its emergency response plan and reported the incident to the relevant authorities in accordance with applicable regulations and procedures. The accident investigation and subsequent follow-up measures are being carried out in an orderly manner. The company and Guang’an Lier will earnestly learn from the lessons of this incident, conduct thorough safety inspections, and strengthen production safety management.
　　At present, Lier Chemical and its other subsidiaries are operating normally. This incident has not had a material impact on the company’s production or operations, nor has it significantly affected its financial performance for the current year.
　　Further Reading
　　In recent years, leaks, fires, and explosions involving reaction vessels and distillation columns have occurred repeatedly. Because these vessels often contain toxic and hazardous chemicals, the consequences of such incidents are far more severe than those of typical explosions. Below, we conduct a comprehensive analysis of the risk factors that contribute to accidents in reaction vessels and distillation columns, and propose corresponding safety measures.
　　Inherent hazard
　　1 Material
　　The materials in reaction vessels and distillation columns are predominantly hazardous chemicals. If the materials have low autoignition temperatures and flash points, a leak can create an explosive mixture with air; upon encountering an ignition source—such as an open flame, sparks, or static electricity—this mixture may result in fire or explosion. Furthermore, if the materials are toxic, a leak could lead to human poisoning or asphyxiation.
　　2 Manufacturing Issues of Equipment and Devices
　　Unreasonable design of reaction vessels and distillation columns, discontinuities in equipment geometry, and improper weld‑seam placement can all lead to stress concentrations. Inappropriate material selection, substandard welding quality during fabrication, and inadequate heat treatment may reduce material toughness. Furthermore, corrosion by aggressive media, loss of structural integrity, or the absence of safety devices can all increase the risk of vessel rupture or explosion during operation.
　　Operational Hazard
　　1. Fire and explosion caused by runaway reaction
　　Many chemical reactions—such as oxidation, chlorination, nitration, and polymerization—are highly exothermic. If the reaction becomes uncontrolled or is suddenly interrupted by a power or water outage, heat can accumulate, causing the temperature inside the reactor to rise sharply and the pressure to increase beyond its pressure‑bearing capacity, potentially leading to vessel rupture. Material ejected from the rupture may ignite, resulting in fire and explosion hazards. Furthermore, the rupture disrupts the equilibrium of vapor pressure within the reactor; the ensuing unstable superheated liquid can trigger a secondary explosion (a steam explosion). The discharged material then rapidly disperses, enveloping the area around the reactor in droplets or vapors of flammable liquids, which, upon encountering an ignition source, may cause a third explosion—a mixture‑gas explosion.
　　The primary causes of runaway reactions include inadequate removal of reaction heat, non-uniform dispersion of reactants, and operational errors.
　　2. High-pressure materials in the reaction vessel surge into the low-pressure system, causing an explosion.
　　Equipment operating at atmospheric or low pressure and connected to the reaction vessel may experience a physical vessel rupture if high-pressure process fluids surge into it, exceeding the vessel’s pressure‑bearing limit.
　　3. Accidents caused by the ingress of water vapor or water into the reaction vessel
　　If steam or heat-transfer oil used for heating, or cooling water, leaks into a reactor or distillation kettle, it may react with the materials inside, decompose and release heat, leading to a rapid rise in temperature and pressure, causing the materials to be ejected and resulting in a fire.
　　4. An explosion occurred due to the absence of cooling water in the distillation–condensation system.
　　During the distillation process, if the cooling water to the overhead condenser is interrupted while the material in the still pot continues to circulate, the system may transition from its normal atmospheric or vacuum condition to a positive pressure. This can exceed the equipment’s pressure‑bearing capacity and lead to an explosion.
　　5 Explosion accidents caused by heating of containers
　　Due to ignition of external combustible materials or exposure to thermal radiation from high‑temperature heat sources, the temperature inside the reaction vessel may rise sharply, leading to a pressure increase and resulting in material ejection or an explosion.
　　6. Accidents caused by improper handling of materials entering and exiting containers
　　Many Class A flammable liquids with low flash points are transferred into reaction vessels and distillation columns via liquid pumps or vacuum extraction. Most of these materials are insulating and have poor electrical conductivity; if the flow rate is too high, accumulated static electricity cannot be dissipated promptly, potentially leading to fires or explosions.
　　7. The workers became complacent and failed to promptly identify early warning signs of an accident.
　　Reactor vessels typically operate at atmospheric pressure or with an open top, while distillation columns are generally run at atmospheric pressure or under vacuum. Some people believe that operating at atmospheric pressure—whether with an open top or under vacuum—poses little risk, leading to complacency and a failure to promptly detect and address early signs of emergencies, which can ultimately result in accidents. In reality, reactor vessels operating at atmospheric pressure or with an open top subject their vessel walls to higher internal pressures than pressurized reactors, making them inherently more hazardous.
　　For distillation columns, if operators make an error and the reaction becomes uncontrollable, leading to blockages in the piping and valve system, the normal atmospheric or vacuum conditions can rapidly transition to positive pressure. If such a situation is not promptly detected and addressed, and the equipment lacks an emergency pressure-relief device, fire and explosion accidents are highly likely to occur.
　　Safety Measures
　　The primary function of a reactor is to facilitate processes such as the polymerization of raw materials, enabling efficient production and achieving optimal outcomes. However, it is essential to adhere to standardized operating procedures; improper handling can lead to equipment damage and disrupt the production process.
　　1. Operate strictly in accordance with the established procedures.
　　Strictly adhering to standardized operating procedures is the most fundamental requirement. Before operating a chemical reactor, it is essential to familiarize yourself with the equipment’s prescribed operating guidelines. Although the equipment manual may not provide highly detailed instructions, certain mandatory protocols must be followed to ensure safe operation.
　　2 Pre-Operation Inspection
　　Before operating a chemical reactor, perform a thorough inspection to verify that the equipment is in proper condition. If the equipment is running normally, never open the top cover or the access plate to prevent electric shock. Never operate under pressure; doing so can damage the equipment and pose serious safety risks. During nitrogen pressure testing, closely monitor pressure changes to avoid excessive pressure.
　　3. Pay close attention to observation
　　When operating a chemical reactor, pay close attention and carefully monitor each step of the procedure. In particular, once the reactor has been heated to a stable temperature, avoid any direct contact with the vessel to prevent burns. After completing the experiment, begin by cooling the system and allow the temperature to drop gradually; overheating can damage the equipment. Additionally, be sure to disconnect the power supply promptly.
　　4. Please take good care of it.
　　Equipment operation requires attention not only to the procedural steps but also to routine maintenance. Proper maintenance is no simple task; only by mastering it can equipment perform at its best and enjoy a longer service life. Conversely, neglecting maintenance can severely compromise operational performance.
　　Key points to note
　　To prevent fires and explosions in reaction vessels and distillation columns, in addition to strengthening safety training and on-site safety management, conducting regular equipment maintenance, preventing the formation of explosive mixtures, promptly removing scale from equipment and piping, carefully controlling feed and discharge flow rates, and using explosion-proof electrical equipment with proper grounding, it is also essential to strictly adhere to established safety operating procedures and job-specific safety protocols.
　　During distillation, process parameters such as temperature, pressure, feed rate, and reflux ratio must be strictly controlled. When using steam heating, the valve opening should be appropriately adjusted to prevent excessive or abrupt opening, which could cause rapid evaporation of the feed and a sharp increase in system pressure.
　　Always ensure that the equipment and piping of the distillation system remain unobstructed, preventing blockages in inlet and outlet lines and valves that could lead to pressure buildup and pose a safety hazard. Avoid allowing low-boiling substances or water to enter the high‑temperature distillation system; prior to startup, thoroughly drain all condensate from the reactor, column, and associated equipment to prevent sudden contact with hot process streams, which could cause instantaneous vaporization and pressure surges, resulting in material spattering or an explosion.
　　Summary
　　Reactor vessels and distillation columns shall be equipped with comprehensive instrumentation for monitoring temperature, pressure, flow rate, and other parameters. For vacuum distillation, the vacuum pump must be fitted with a check valve to prevent atmospheric air from entering the system in the event of an abrupt shutdown. A check valve should also be installed at the interface between low-pressure and high-pressure systems to prevent material from high-pressure vessels from backflowing into the low-pressure system and causing an explosion. Reactor vessels and distillation columns that may experience overpressure must be provided with emergency pressure-relief devices; typically, safety valves are installed on the equipment. For equipment where safety valves are unsuitable or where the process poses significant hazards, rupture discs may be used instead.

Author: Lier Chemical Announcement, Chemical 707, etc. Editor: xwbj1